Why we need a space station

With all the hoopla over the International Space Station (aka Freedom, just to piss off the Russians), I've noticed that whenever anyone official is asked why we're building it their answers tend to take the form of lists.
Also notable is that always, buried among the other reasons and usually near the bottom, is my favorite reason of all: Orbital Transfer Vehicles.

If we're ever gonna explore our Solar System, much less go beyond it, we need to use spacecraft that we can assemble in orbit. The physics of escape velocity mean that there is no way to boost enough mass off the planet to have enough fuel and consumables left to get anywhere meaningful. The most efficient means of doing this is to use the Space Shuttle and its successors to boost supplies and construction materials up the gravity well to LEO, which is what they were/are/will be designed for, and then build completely exoatmospheric vehicles designed to go further. Engines designed only for space can be incredibly more efficient; in rocket terms, this is called having a high Isp (pronounced eye-ess-pee). This stands for Specific Impulse, and is a standardized measurement of a reaction drive's efficiency.

Engines using exotic techology such as Bussard ramjets, Ion Engines, solar sails and others cannot be used to leave Earth, since they are far too weak (despite being more efficient!). However, once in space, where escape velocity doesn't have the immediacy it does during launch, these engines can be used to accelerate gently for long periods of time. Vehicles thus built would be able to go to Mars or even the outer planets on less fuel than it took to lift their components into orbit!